1. Field of the Invention
The present invention relates to a color cathode ray tube of a shadow mask type, and more particularly, to a highly reliable color cathode ray tube which can enhance the material strength of a shadow mask, can improve the etching characteristic and formability, and can decrease the thermal deformation.
2. Description of the Related Art
With respect to color cathode ray tubes which are used as monitor devices for information equipment or display means of color picture receivers of recent year, a flat face technique which makes a panel (face panel) constituting an image display surface flat has been rapidly spreading. Particularly, when a shadow mask of a press forming type which has an apertured surface thereof curved in the horizontal direction and in the vertical direction is adopted, the panel of the color cathode ray tube (flat-face tube) having the flat face has an approximately planar outer surface and an inner surface which has a curvature considerably larger than that of the outer surface.
As one of the technical tasks at the time of designing such a flat face tube, the shadow mask strength is named. Although the shadow mask is formed with a curvature which approximates the curvature of the inner surface of the panel, the flat face tube has the small curvature of inner surface of the panel compared with a round face tube which has both of inner and outer surfaces thereof curved and hence, there is no other way but to make the curvature of the shadow mask of the flat face tube also small. Accordingly, it is difficult to maintain the strength against the thermal deformation generated by a so-called doming phenomenon of the apertured surface of the shadow mask which is caused by the elevation of the temperature of the shadow mask upon impingement of electron beams in operation. Further, it is also difficult to maintain the physical strength against the falling of the shadow mask, the shock or the like.
As the material which ensures the strength of the shadow mask of this type, cobalt-doped Invar material which is produced by doping cobalt in conventional Invar material has been used.
Although aluminum killed steel material was used as the shadow mask material, recently Invar material has been used along with efforts to enhance the definition of the images and to make the screen further flat. The cobalt-added Invar material which is used for enhancing the strength of the shadow mask ensures the approximately 20% increase of the strength compared with the usual Invar material and can suppress the above-mentioned deformation of the shadow mask. However, the shadow mask produced by doping cobalt in the Invar material has several defects including (1) the cost is pushed up since the cobalt is expensive, (2) the etching efficiency is decreased since the erosion resistance of cobalt is favorable, (3) the workability is decreased and (4) the magnetic characteristics is decreased.
Accordingly, it is an object of the present invention to provide a flat-face type color cathode ray tube having a shadow mask which can decrease drawbacks which such a conventional shadow mask suffers from.
A typical gist of the present invention to achieve the above-mentioned object lies in that a shadow mask material which is served for a color cathode ray tube is made of an iron material which is formed of a plate body doped with an alloy element and has a concentration gradient of the alloy element which is gradually decreased from one surface of the plate body to the other surface of the plate body. Typical constitutions of the present invention are as follows.
(1) In a color cathode ray tube having an evacuated envelope which includes a panel which coats phosphor of a plurality of colors on an inner surface thereof, a neck which houses an electron gun and a funnel which connects the panel and the neck, and a shadow mask which is installed close to the phosphor coated on the inner surface of the panel and has a large number of color selection apertures, wherein
material which constitutes the shadow mask is formed of a single plate body made of an iron material containing an alloy element and the single plate body is a composite gradient alloy plate having a concentration gradient of the alloy element which is continuously decreased from a phosphor-side surface to an electron-gun-side surface.
(2) When the signal plate body which constitutes the composite gradient alloy plate contains nickel as the alloy element, the content of nickel in the vicinity of the phosphor-side surface of the single plate body is set to not more than 45 wt % and the content of nickel in the vicinity of the electron-gun-side surface of the single plate body is set to not less than 0 wt %.
(3) In the above-mentioned case (2) in which the signal plate body which constitutes the composite gradient alloy plate contains nickel as the alloy element, the content of nickel in the vicinity of the phosphor-side surface of the single plate body is set to not more than 36 wt % and the content of nickel in the vicinity of the electron-gun-side surface of the single plate body is set to not less than 16 wt %.
(4) The color selection apertures formed in the shadow mask are formed in a dot array and the whole shadow mask is formed of a type which maintains a shape thereof by itself.
(5) The color selection apertures formed in the shadow mask are formed like a plurality of continuous lines extending in one direction and tension is applied to a member which forms the line-like apertures in the extension direction of the apertures.
(6) The color selection apertures formed in the shadow mask are formed of a plurality of slots having long axes thereof extended in one direction and the whole shadow mask is formed of a type which maintains a shape thereof by itself or tension is applied to the shadow mask in the long-axis direction of the slot-like apertures or in the long-axis direction as well as in the short-axis direction.
The present invention is not limited to the above-mentioned constitutions and structures which will be explained in embodiments described later and various modification may be conceivable without departing from the technical concept of the present invention.